1. Field of the Invention
This invention relates to a copy guard processing detecting apparatus for detecting whether or not a video signal of a recorded video tape on which a piece of video software such as a motion picture is recorded is processed for prevention of duplication of itself.
2. Description of the Related Art
It is a popular practice, in order to make it substantially impossible to copy or duplicate a recorded tape, on which a piece of video software such as a motion picture is recorded, by means of a video tape recorder for home use, to apply some processing to the recorded video signal for prevention of duplication of the signal itself (such processing will be hereinafter referred to as copy guard).
An exemplary one of conventional copy guard methods is disclosed in Japanese Patent Laid-Open Application No. 61-288582 and is popularly used. According to the method, a plurality of pairs of pulses each including a false synchronization pulse and a positive pulse are inserted in each vertical blanking interval, and an AGC (automatic gain control) circuit of the recording system of a predetermined characteristic is utilized for processing with a copy guard. The reason why a signal for a copy guard is inserted in a vertical blanking interval is that the signal for a copy guard may possibly have a bad influence on contents themselves of a piece of video software otherwise if it is inserted directly into an image signal interval.
FIG. 4 illustrates the copy guard method disclosed in the document mentioned above. The waveform B in FIG. 4 shows an example of a video signal wherein a plurality of pairs of pulses each including a false synchronization pulse and a positive pulse for a copy guard (such pairs of pulses will be hereinafter referred to as copy guard signal) are inserted in a vertical blanking interval. Meanwhile, the waveform A in FIG. 4 shows another video signal which does not include such pulses inserted therein. In the video signal shown by the waveform of FIG. 4, five pairs of pulses each including a false synchronization pulse DP and a positive pulse LP are inserted for each fixed period in each of horizontal intervals from the thirteenth to eighteenth horizontal intervals in a vertical blanking interval of an odd-numbered field.
It is to be noted that, in the case of the video signal shown by the waveform B of FIG. 4, though not shown, another copy guard signal is inserted similarly between the 275th to 280th horizontal intervals of an even-numbered field.
In the video signal of the waveform B of FIG. 4, a rising edge of a false synchronization pulse DP continues to a rising edge of a next positive pulse LP, and accordingly, if the copy guard signal is inserted in the video signal, an AGC circuit of the recording system of a video tape recorder will determine that a synchronization pulse has a magnitude equal to the magnitude from a peak value of the false synchronization pulse DP to a peak value of the positive pulse LP and accordingly will effect gain control as if the level of the input video signal is as high as, for example, three times or more a standard level. Consequently, if the recorded signal is reproduced, then since it has been attenuated to a level equal to or less than 30% of the standard level, only an unstable image of a low quality can be obtained. In other words, a copy guard can substantially be achieved.
Also it is a common practice to change the level of such positive pulses LP suitably to change the AGC level to various values in order to further deteriorate the quality of the reproduced image.
Since the conventional copy guard coping processing relies on a peculiar characteristic of an AGC circuit of a video tape recorder with regard to an external input, the copy guard function cannot sometimes be obtained sufficiently depending upon the type, the characteristic or the performance of the AGC circuit.
Further, since the conventional copy guard method does not cause the AGC circuit to operate in accordance with a result of judgment whether or not a copy guard is actually applied to a video signal, it has a drawback that the AGC circuit reacts with a noise which is generated when, for example, tracking is lost or variable speed reproduction is performed with the video tape recorder or with some other disturbance noise taking the noise as a false pulse so that the video signal to be recorded is deteriorated to a low quality although the video signal is originally a normal video signal.
The drawback arises from the fact that the copy guard coping processing function is normally put into an operating condition whether or not a copy guard is applied to a video signal.